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Re: [Qemu-discuss] [libvirt] vm live storage migration with snapshots

From: Eric Blake
Subject: Re: [Qemu-discuss] [libvirt] vm live storage migration with snapshots
Date: Wed, 11 Feb 2015 10:52:49 -0700
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On 02/11/2015 10:08 AM, Edward Young wrote:
> Hi all,

[probably didn't need to cross-post to quite that wide of an audience,
oh well]

> I'm investigating the ways to improve the live migration performance in
> libvirt. I have a question about the vm live storage migration. the
> platform is libvirt + qemu + kvm
> If we want to migrate a running vm with its virtual disk images to another
> node.
> we can use 'virsh migrate ....' commands.
> What if this vm has a number of disk-only external snapshots? In the
> current version, how can live migrate this vm?

Are the snapshots based on shared storage, or local-only storage?

If I understand your question correctly, you are starting with this

base <- mid <- active

and want to make sure that on the destination, the guest still sees the
same contents as 'active' on the source, whether or not the destination
still sees a backing chain or just one file.

> Is it possible to blockcommit the snapshots to the base image and later
> perform the migration, without shutting down the running vm?

Yes, but that may not be the fastest solution.  That is, you would be
going from:

base <- mid <- active



where base' is the active image containing all the committed state, then
migrate that file.  If base is not shared, that means qemu has to
migrate the entire disk state.  And you also no longer have the external
snapshots to revert to on the destination.

> Or is it possible to iteratively transfer all the snapshots to the
> destination and later live migrate only the latest new data?

Yes, that works too, and is probably faster, especially if you have
out-of-band means for sharing read-only state between source and

In fact, if you can create yet another (temporary) external snapshot,
but this time on shared storage, and have storage backends that let you
instantly remap backing files to be visible into the destination, then
it is as simple as:

create a snapshot with shared storage destination
base <- mid <- active(frozen) <- shared
copy base, mid, and active to the destination
live migrate using shared storage instead of doing storage migration
block-commit shared back into active
base <- mid <- active

where the shared storage only needs to hold as much disk state as
diverges during the time of the live migration.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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